DESCRIPTION (Verbatim from the Applicant's Abstract): Phase variation is a general term for heritable, metastable gene expression states. Phase variation has been described in bacteria and fungi and is associated with pathogenesis. Colony morphology switching has been extensively studied in Candida albicans because it is considered to be a pathogenesis trait, but genetic analysis has been difficult in this system. The PI has discovered that phase variation occurs in pathogenic strains of Saccharomyces cerevisiae isolated from human patients, but not in avirulent laboratory strains. Extensive genetic characterization of one phase identity locus (PHI) has shown that it is most likely due to a tRNA ochre suppressor that switches between two states, PHI1-1 and PHI1 -2, representing suppressive and non suppressive states, respectively. The second form of phase variation described by the investigator is switching between mannitol utilization states. Cloning the PHI loci by complementation is complicated by the fact that the states are metastable. Instead, these loci will be cloned by a positional cloning strategy using technology developed in the Davis lab. Once the PHI genes are identified, sequencing of the different alleles might provide clues to the mechanism of switching. The PHI genes will be introduced to the non-phase variable laboratory strain, 288c, in efforts to reconstruct phase variation.